Smithsonian.com

Did Life Come to Earth From Mars?

Mineralogical clues point to the idea that the early Earth, starved of oxygen and submerged by a vast ocean, needed molecules from Mars to kick start life

By
Natasha Geiling

smithsonian.com
August 28, 2013

Could life on Earth have been born on Mars? Photo via Wikipedia.

If the phenomena of Star Trek, Area 51, Ancient Aliens, or War of the Worlds can be taken as anthropological clues, humanity is consumed with curiosity about the possibility of life beyond Earth. Do any of the 4,437 newly discovered extrasolar planets contain traces of life? What would these life forms look like? How would they function? If they came to Earth, would we share ET-esque embraces or would the visit be more a Battle Los Angeles style throw down?

Life outside of Earth has spawned endless interest, but less public interest seems to be given to how life on Earth began 3 to 4 billion years ago. But the two topics, it turns out, might be more connected than one would believe–in fact, it’s possible that life on Earth really began outside of Earth, on Mars.

The first, the tar paradox, is simple enough to understand. “If you put energy into organic material it turns to asphalt, not to life,” Benner explains. Without access to Darwinian evolution–that is, without organic molecules having the opportunity to reproduce and create offspring who themselves, mutations and all, are reproducible–organic matter that is bathed in energy (from sunlight or from geothermal heat) will turn into tar. Early Earth was full of organic materials–chains of carbon, hydrogen and nitrogen that are believed to be the building blocks of life. Given the tar paradox, these organic materials should have devolved into asphalt. “The question is, how is it possible that the organic materials on early Earth managed to leap from their asphaltic fate to something that had access to Darwinian evolution? Because once that happens–presumably–you’re off to the races, and then you can manage whatever environment you want,” Benner explains.

The second paradox is the so-called water paradox. The water paradox states that even though life needs water, if organic material could escape its asphaltic fate and move toward Darwinian evolution, you can’t assemble the necessary building blocks in a flood of water. The building blocks of life start with genetic polymers–the well-known player DNA and its less-famous but still very smart friend RNA. Experts agree that RNA was likely the first genetic polymer, partly because in the modern world, RNA plays such an important role in the manufacturing other organic compounds. “RNA is the key to the ribosome, which is what makes proteins. There’s almost no question that RNA, which is a molecule involved in catalysis, arose before proteins arose,” Benner explains. The difficulty is that for RNA to assemble into long strands–which is needed for genetics–you can’t have the assembly taking place in water. “Most people think that water is essential for life. Very few people understand how corrosive water is,” Benner says. For RNA, water is extremely corrosive–bonds cannot be made within water, preventing long-strands from forming.

However, Benner says that these paradoxes can be resolved with the help of two very important groups of minerals. The first are borate minerals. Borate minerals–which contain the element boron–prevent life’s building blocks from devolving into tar if incorporated into organic compounds. Boron, as an element, is seeking electrons to make itself stable. It finds these in oxygen, and together the oxygen and boron form the mineral borate. But if the oxygen boron finds is already bonded to carbohydrates, the carbohydrates linked with boron form a complex organic molecule dotted with borate that’s less resistant to decomposition.

Which brings us–however circuitously–back to Mars. Both borate and molybdate are scarce and would have been especially scarce on early Earth. The molybdenum in molybdate is highly oxidized, meaning that it needs electrons from oxygen or other readily available negatively charged ions to achieve stability. But early Earth was too oxygen-scarce to have readily created molybdate. Plus, returning to the water paradox, early Earth was quite literally a water world–with land making up only two to three percent of its surface. Borates are soluble in water–if early Earth was a flooded planet, as scientists believe, it would have been difficult for an already scarce element now diluted in a huge ocean to find ephemeral organic molecules to bond with. Moreover, Earth’s status as a water-logged planet makes it difficult for RNA to form, because that process can’t easily happen in water on its own.

And, Benner believes, molybdate was there too. “It’s only when molybdenum becomes highly oxidized that it is able to influence how early life formed,”Benner explains. “Molybdate couldn’t have been available on Earth at the time life first began, because three billion years ago the surface of the Earth had very little oxygen, but Mars did.”

Benner believes that these factors imply that life originated on Mars, our closest neighbor in space equipped with all the right ingredients. But life wasn’t sustained there. “Of course Mars dried out. The process of drying was very important for life originating, but not sustaining,” Benner explains. Instead, a meteor would have to have hit Mars, projecting materials into space–and eventually those materials, including some building blocks of life, might have made it to Earth.

Would the sudden change in environment have been too harsh for the fledgling building blocks to survive? Benner doesn’t think so. “Let’s say life starts on Mars, and becomes very happy in the Martian environment,” Benner explains. “A meteor comes to hit Mars, and the impact ejects rocks on which your predecessor is sitting. Then you land on Earth, and you discover that there is lots of water that you were treating as a scarce element. Will it find the environment adequate? It certainly appreciated the existence of enough water that it didn’t have to worry.”